It is desired to have a process to increase gasoline production by consuming low value isobutane and pentenes and producing a high quality alkylate gasoline. Others have tried alkylating isobutane with pentenes to improve gasoline production and reduce evaporative hydrocarbon emissions, but the processes using either hydrofluoric or sulfuric acid alkylation catalysts have had significant shortcomings. Hydrofluoric (HF) and sulfuric acid (H2SO4) alkylation catalysts produce significant amounts of isopentane and/or n-pentane during the alkylation of pentenes, and both isopentane and n-pentane have undesirably high Reid Vapor Pressures.
Alkylate gasoline is a highly desirable blending component for motor gasoline with its high octane, low sulfur level and no aromatics. As the gasoline specifications have become tightened worldwide due to heightened environmental concerns, the demand for increased use of alkylate gasoline has been increasing steadily over the years. Most of the refineries in United States already have isoparaffin alkylation units that alkylate isobutane with C3 and C4 olefins from fluid catalytic cracker (FCC) units to produce alkylate gasoline. The FCC units also produce a light gasoline fraction, containing substantial amount of C5-C6 olefins, and due to the difficulties in performing alkylations with them, they are currently blended into the gasoline pool.
Alkylation of C5 olefins has been practiced to only a limited extent in the industry so far. There is a trend suggesting that it would be desired to have more of the C5 olefins be alkylated in the future to help meet tighter environmental regulations. There remains a substantial need to develop an efficient alkylation process for C5 olefins.
Conventional alkylation processes using HF catalyst are not effective in alkylating C5 olefins. During the pentenes alkylation with isobutane, the HF alkylation process generates substantial amounts of isopentane through a hydrogen transfer reaction. This is highly undesirable due to the high Reid Vapor Pressure (RVP) of isopentane (21 RVP for isopentane vs. 6-7 maximum RVP specification for gasoline). An example of the high RVP issue with the HF alkylation process is shown in U.S. Pat. No. 5,382,744 by Abbott el al., which showed that alkylation of 2-methly-2-butene converts about 70 mol % of the pentene to isopentane via hydrogen transfer. In the patent, Abbott et al. found that recycling of isopentane to the reactor reduces the isopentane formation somewhat. However, the recycling of isopentane would make the overall process complicated and raise the cost.
Conventional alkylation processes using H2SO4 catalysts are less susceptible to the hydrogen transfer than processes using HF catalyst, but still from about 0% up to 20% of C5 olefins are converted to isopentane during the alkylation process using H2SO4 catalyst (Stratco report, published in 1999 by Randall Peterson, David Graves, Ken Kranz and David Buckler). The H2SO4 alkylation process for alkylating C5 olefins also increases the acid consumption. The H2SO4 alkylation processes are highly susceptible to cyclopentene and diene contaminants. The cyclopentene, dienes and other contaminants further increase the acid consumption to an even higher level. To control the acid consumption, a very low reaction temperature of less than 10° C. may need to be used for the processes using H2SO4 catalyst, which requires additional equipment and higher cost.